Analysis on Surface Morphology Formation Process of Al-Si Coated 22mnb5 Steel During Laser Welding

A special morphology is formed on the upper surface of an Al-Si coated steel laser welded joint. Due to the uneven mixing of the coating element and the base metal (BM), the brittle phase is formed at the fusion line on the upper surface of the welded joint, resulting in its mechanical properties not meeting the requirements. In this study, the surface morphology formation process of Al-Si-coated steel laser-welded joints is analyzed. The results show that a wavy structure is formed at the edge of the weld under the action of the Karman vortex street. On the one hand, a multiphase microstructure of δ-ferrite, lath martensite (LM) and Fe-Al intermetallic compounds (Fe-Al IMCs) is formed at the convexity of the wavy structure. The brittle phase of the fusion line at the upper surface of the welded joint is δ-ferrite. On the other hand, since the melting point of the Al-Si coating at the concavity (between the adjacent convexity) is much lower than the temperature of the surrounding liquid metal, the Al-Si coating at this position is melted into the molten pool under the action of heat conduction and flows to the weld center under the action of gravity, forming river-like Fe-Al IMCs on the surface of the weld. By removing the coating layer or reducing the welding speed, the formation of a Karman vortex street can be effectively avoided, and finally, δ-ferrite precipitation at the weld surface fusion line can be inhibited.